Process to regenerate fcc spent catalyst

ABSTRACT

A process to supply solid particles to a fluidized bed via a riser which has a substantially vertical upper part terminating at an outlet opening in the fluidized bed and wherein the solid particles are transported towards the fluidized bed in the riser with a lift gas, which lift gas and solid particles are contacted at an upstream part of the riser and wherein between the upstream part and the outlet opening the interior of the riser is provided with a plurality of axially spaced mixing elements.

The present invention is related to a Process to supply solid particles to a fluidized bed via a riser which has a substantially vertical upper part terminating at an outlet opening in said fluidized bed and wherein the solid particles are transported towards said fluidized bed in the riser with a lift gas, which lift gas and solid particles are contacted at an upstream part of the riser. The invention is especially directed to a process for improving distributions of both spent catalyst and lift gas into a regenerator of a fluid catalytic cracking unit.

In a typical Fluid Catalytic Cracking Unit (FCCU) consisting of a regenerator, a riser reactor and a stripper, finely divided regenerated catalyst is drawn from the regenerator through the regenerator standpipe and contacts with a hydrocarbon feedstock in a lower portion of a reactor riser. Hydrocarbon feedstock and steam enter the riser through feed nozzles. The mixture of feed, steam and regenerated catalyst, which has a temperature of from about 200° C. to about 700° C., passes up through the riser reactor, converting the feed into lighter products while a coke layer deposits on the surface of the catalyst, temporarily deactivating the catalyst. The hydrocarbon vapours and catalyst from the top of the riser are then passed through cyclones to separate spent catalyst from the hydrocarbon vapour product stream. The spent catalyst enters the stripper where steam is introduced to remove hydrocarbon products from the catalyst. The spent catalyst then passes through a spent catalyst transfer line to enter the regenerator where, in the presence of air and at a temperature of from about 620° C. to about 760° C., the coke layer on the spent catalyst is combusted to restore the catalyst activity. Regeneration is performed in a fluidized bed. The regenerated catalyst is then drawn from the regenerator fluidized bed through the regenerator standpipe and, in repetition of the previously mentioned cycle, contacts the feedstock in the reactor riser.

Catalyst regeneration is a critical step in FCCU operation. The success of the step depends on the contacting efficiency between the spent catalyst and oxygen-containing gas in the regenerator. While the operation of an FCCU with a single catalyst inlet opening was acceptable for many years, the potential benefit of improving catalyst distribution in the regenerator has become apparent more recently. An ideal condition for catalyst distribution is that the time for distribution and mixing of catalyst should be less than that for coke combustion. As the regenerator diameter increases, the radial mixing time of catalyst becomes longer. At the same time, as the regeneration temperature increases, the time required for combustion becomes shorter. Hence, the benefit of improving spent catalyst distribution is more significant for an FCCU comprising a regenerator vessel of large diameter or in which regeneration is conducted at higher temperature.

Another important aspect of the spent catalyst distribution is to control afterburn, which is characterized by substantial temperature increase in the dilute phase of the regenerator above the fluidized bed. If the lift gas, most commonly air, coming along with the spent catalyst is not well distributed, gas will form large bubbles at the discharge of the spent catalyst distributor, rising quickly through the fluidized bed with little time for combustion, and releasing oxygen-rich gas into the dilute phase. This leads to afterburn and poor combustion efficiency of the transport gas in the fluidized bed.

EP-A-622116 discloses a device which distributes spent catalyst and lift gas by means of a central vertical spent catalyst riser terminated with a junction connecting to multiple, horizontal conveying conduits and discharging catalyst at the ends of the horizontal conduits to discrete distribution points.

Although the above prior art device is directed at distributing spent catalyst as best as possible within the regenerator there is room for improvement in this respect.

It is an objective of the instant invention to improve solids distribution in a fluidized bed when supplying said solids. Another objective is to achieve this improvement while making use of the above and other prior art devices to supply spent catalyst to the fluidized bed wherein the device comprises a riser with a substantially vertical upper part.

This objective is achieved with the following process. Process to supply solid particles to a fluidized bed via a riser which has a substantially vertical upper part terminating at an outlet opening in said fluidized bed and wherein the solid particles are transported towards said fluidized bed in the riser with a lift gas, which lift gas and solid particles are contacted at an upstream part of the riser and wherein between said upstream part and the outlet opening the interior of the riser is provided with a plurality of axially spaced mixing elements.

Applicants have found that when such mixing elements are present in the riser according to the process of the invention an improved distribution of solids and lift gas in the fluidized bed is achieved. Furthermore the improvement can be achieved by making use of the spent catalyst inlet devices of the prior art. Thus by simple retrofitting the riser of these prior art devices an improved distribution can be achieved.

The mixing elements, which are present on the surface of the interior of the riser, will result in that the solids and gas will flow more evenly upwards through the riser. It has been found surprisingly that this more evenly flow through the whole length of the riser, which can range from 6 to 30 meters, would have such a major advantageous impact in the distribution of solids and gas in the fluidized bed in which gas and solids are dispersed into.

The mixing elements can be any extension of the interior of the riser wall towards the riser interior resulting in a smaller cross sectional opening than the opening at a point where no mixing element is present. Such a smaller opening will create turbulence and a higher local gas velocity resulting in local radial mixing of the gas-solids mixture flowing in the riser. The axial spacing of the mixing elements is not critical, usually it should be larger than the diameter of the riser. Examples of such devices are venturi shaped rings fixed on the interior of the riser. If a refractory is present in the riser it is preferred to use specially shaped refractory building blocks as mixing element. An example of such a mixing element is disclosed in U.S. Pat. No. 3,353,925 and U.S. Pat. No. 5,851,380 for a FCC reactor riser.

Preferably the mixing element has the shape of a segment of arc as described in WO-A-9814533. These mixing elements are preferred over the elements disclosed in U.S. Pat. No. 3,353,925 and U.S. Pat. No. 5,851,380 because the pressure drop over the length of the riser provided with such elements is lower. The mixing elements have the shape of a segment of arc and a rectangular cross-section. The horizontal penetration depth of the flat mixing element towards the center of the riser is equal to the rise of the segment of arc.

Suitably one to at most four arc shaped mixing elements can be positioned at a specific elevation in the riser. The arc shaped mixing elements can be arranged directly above another in the riser. However, suitably the arc shaped mixing element(s) is (are) arranged staggered with respect to the arc shaped mixing element(s) positioned just above or below said elements. For example if two opposite arc shaped mixing elements are used per elevation the adjacent mixing elements are suitably arranged perpendicular to each other. The angle need not be 90°, it can be any acute angle.

In addition, the flat mixing element(s) can further include a lip pointing inwards.

The central planes of the flat mixing elements 10 are arranged perpendicular to the central longitudinal axis of the riser. In an alternative embodiment the mixing elements can be arranged tilted downwards with respect to the central longitudinal axis of the riser 1. For example the angle between a central plane and the central longitudinal axis is between 5° and 20°.

In the riser a dilute phase fluidized bed will be present. Preferably the more dilute phase fluidized bed has a catalyst or solids density of below 500 kg/m³ and more preferably between 20 and 400 kg/m³. The superficial gas velocity in the more dilute phase fluidized bed is preferably between 1.5 and 20 m/s.

At the upper end of the riser means to distribute the solids and the lift gas in the dense fluidized bed are suitably present. Such means may for example be a device as described in the above referred to EP-A-622116 or similar devices comprising outwardly conveying conduits to distribute the solids. A device found especially suitable is described in WO-A-242394 and is incorporated herein by reference. Such device comprises of:

a first disk surrounding the upper opening of the vertical riser at the uppermost end of said riser;

a second disk, spaced upwardly from, and rigidly connected to, said first disk, thereby forming a substantially open space therebetween;

a deflection cone attached, at its base, to said second disk, said deflection cone pointing downward and being centred over the outlet of said conduit, said deflection cone adapted to direct said spent catalyst and said transfer gas in a substantially uniform, radially outward direction through said space formed between said first disk and said second disk, thereby providing a continuous circumferential discharge of said mixture of solids and lift gas from the outer circumference of said space formed between said first disk and said second disk into said fluidised bed in a substantially uniform radially outward direction.

The lift gas may be any gaseous medium and the choice will depend on the process in which the invention is applied. When spent catalyst solids are supplied to a FCCU regenerator the lift gas may be for example steam or nitrogen. Preferably an oxygen containing gas, suitably air or oxygen-enriched air is used.

When the process according to the present invention is used in a FCCU the solids will be catalyst suitably be conventional FCC catalyst as for example described in “Fluid catalytic cracking: Science and Technology”, Ed. Magee J. S., Mitchell M. M. Jr., 1993, Elsevier Science Publishers B. V., pages 1-6. Additives, for example ZSM-5 or Zeolite Beta containing additives, to enhance propene selectivity may also be present.

The invention shall be further illustrated by means of FIG. 1.

FIG. 1 shows the lower end of a FCCU regenerator vessel (1) comprising of a fluidized bed (2) of catalyst particles, an upper fluidized bed level (3), means to add fluidizing gas (4), a catalyst discharge conduit (5) for discharge of regenerated catalyst particles and means to supply spent catalyst via a riser (6) having a substantially vertical upper part. Riser (6) is provided at its upper end with means (7) to evenly discharge catalyst and lift gas in the fluidized bed (2). These means are thus located below upper bed level (3). The riser is provided with axially spaced mixing elements (8). At the lower end of said riser (6) spent catalyst, as discharged from a FCC stripper (not shown) via (9), and lift gas as provided at inlet point (10) is mixed. 

1. A process to supply solid particles to a fluidized bed via a riser which has a substantially vertical upper part terminating at an outlet opening in said fluidized bed and wherein the solid particles are transported towards said fluidized bed in the riser with a lift gas, which lift gas and solid particles are contacted at an upstream part of the riser and wherein between said upstream part and the outlet opening the interior of the riser is provided with a plurality of axially spaced mixing elements.
 2. The process of claim 1, wherein the mixing element has the shape of a segment of arc.
 3. The process of claim 2, wherein a first disk surrounds the upper opening of the riser at the uppermost end of said riser; a second disk, is spaced upwardly from, and rigidly connected to, said first disk, thereby forming a substantially open space there-between; a deflection cone is attached, at its base, to said second disk, said deflection cone pointing downward and being centred over the outlet of said conduit, said deflection cone adapted to direct said spent catalyst and said transfer gas in a substantially uniform, radially outward direction through said space formed between said first disk and said second disk, thereby providing a continuous circumferential discharge of said mixture of solids and lift gas from the outer circumference of said space formed between said first disk and said second disk into said fluidised bed in a substantially uniform radially outward direction.
 4. The process of claim 3, wherein the solid particles are FCC catalyst particles, which are supplied to a fluidized regenerator bed.
 5. The process of claim 4, wherein the lift gas is air.
 6. The process of claim 1, wherein a first disk surrounds the upper opening of the riser at the uppermost end of said riser; a second disk, is spaced upwardly from, and rigidly connected to, said first disk, thereby forming a substantially open space there-between, a deflection cone is attached, at its base, to said second disk, said deflection cone pointing downward and being centred over the outlet of said conduit, said deflection cone adapted to direct said spent catalyst and said transfer gas in a substantially uniform, radially outward direction through said space formed between said first disk and said second disk, thereby providing a continuous circumferential discharge of said mixture of solids and lift gas from the outer circumference of said space formed between said first disk and said second disk into said fluidised bed in a substantially uniform radially outward direction.
 7. The process of claim 1, wherein the solid particles are FCC catalyst particles, which are supplied to a; fluidized regenerator bed.
 8. The process of claim 2, wherein the solid particles are FCC catalyst particles, which are supplied to a; fluidized regenerator bed. 